The genome of the rotaviruses, which belong to the reoviridae family, consists of 11 discrete segments (genes) of ds RNA. Its unusual segmented genome structure and the relative ease with which reassortants can be generated in vitro provides a unique opportunity to gain insight into genetic and molecular mechanisms of virus-host interactions through mapping biological function to specific genes. During the course of characterizing a variety of rotaviruses of mammalian and avian origin for serotype by plaque reduction neutralization, several intertypic rotaviruses were identified. During the past year, efforts were made to dissect genetically and delineate, serologically, the basis for this "bridging" phenomenon. The fourth gene product, protein VP3, was shown to be responsible for intertypic "bridging" in several instances. Thus, in addition to VP7, the VP3 protein contains antigenic sites which stimulate and react with neutralizing antibodies. In addition, antigenic and functional analysis of the rotaviral fourth gene product was performed.